In steel, petroleum, chemical, cement, paper and pulp, ceramic engineering, biomass and other industries, a once through steam boiler or the like is used, which generates steam by use of heat of combustion of a fossil fuel such as oil, gas or the like in order to supply a heat source for a heating step such as a drying step, a sterilization step or the like. Recently, energy savings and carbon dioxide emissions reduction are serious problems in industries. It is considered that a heating method using a heat pump reduces carbon dioxide emissions as compared with a conventional method of combusting a fossil fuel to supply heat.
A heat pump is generally used for cooling or heating in an air conditioning device or a hot water supply device.
Chlorofluorocarbon (CFC) or hydrochlorofluorocarbon (HCFC) each containing fluorine and chlorine are conventionally used as a coolant of a heat pump cycle, but use thereof has been stopped step by step from the viewpoint of ozone layer protection. Today, a natural coolant such as ammonia, carbon dioxide or the like and hydrofluorocarbon (HFC), which does not contain a chlorine atom in a molecule, are mainly used.
A heat pump known as a hot water supply device or a steam generation device uses HFC as a coolant. For example, PCT Japanese National Phase Laid-Open Patent Publication No. 2013-525720 discloses a coolant for a high-temperature heat pump, which has a condensation temperature of 70° C. to 150° C. This coolant contains 1,1,1,3,3-pentafluoropropane as a main component and contains 1,1,1,3,3-pentafluoroprobutane at a mass ratio of 20% by mass or less.
However, it is concerned that HFC, which has a high global warming potential (GWP), significantly contributes to the warming. Therefore, hydrofluoroolefin (HFO), which is a fluorine-containing unsaturated compound, is proposed as a substitute coolant having a low global warming potential.
For example, PCT Japanese National Phase Laid-Open Patent Publication No. 2007-535611 discloses using, as a coolant of an air conditioning device for an automobile, a composition containing 2,3,3,3-tetrafluoropropene (HFO) and polyalkyleneglycol (PAG) lubricant.
Japanese Laid-Open Patent Publication No. 2010-47754 discloses using, as a coolant of a low-temperature freezer, a mixed compound of tetrafluoropropene such as 2,3,3,3-tetrafluoropropene, 1,3,3,3-tetrafluoropropene or the like and difluoromethane.